As a detector for a gas chromatograph, various types of detectors have been practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not sufficiently low; (2) Its ionization efficiency for alcohols, aromatic substances, and chlorine substances is low; (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system more difficult to operate. On the other hand, as a detector capable of high-sensitivity detection of various compounds from inorganic substances to low-boiling organic compounds, a pulsed discharge detector (PDD) has conventionally been known (for example, refer to Patent Document 1). In the PDD, the molecules of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from their excited state to the ground state, they generate optical energy. This optical energy is utilized to ionize a molecule to be analyzed, and an ion current produced by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the ND for propane is no higher than 0.0005%, whereas the PDD can achieve a level as high as 0.07%. Despite this advantage, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is lower than the latter by one or more orders of magnitude. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, refer to Patent Document 2). This detector uses a low-frequency AC-excited dielectric barrier discharge (which is hereinafter referred to as the “low-frequency barrier discharge”) to create a stable and steady state of plasma. The plasma created by the low-frequency barrier discharge is non-equilibrium atmospheric pressure plasma, which does not become hot as easily as the plasma created by the radio-frequency discharge. Furthermore, the periodic fluctuation of the plasma, which occurs due to the transition of the voltage application state if the plasma is created by the pulsed high-voltage excitation, is prevented, so that a stable and steady state of plasma can be easily obtained. Based on these findings, the present inventors have conducted various kinds of research on the discharge ionization current detector using a low-frequency barrier discharge and have made many proposals on this technique (for example, refer to Patent Document 3 and Non-Patent Documents 1 and 2).
As just explained, the low-frequency barrier discharge can create a stable state of plasma and hence is generally advantageous for noise reduction. However, it is difficult to completely eliminate influences of electromagnetic noises that enter the ion-collecting electrode. It is also difficult to prevent the detection signal from a drift due to the fluctuation in the ambient temperature around the detection cell, which may be heated up to approximately 400 degrees Celsius for the detection of high-boiling components. In the case of a detector for GC or similar detector that is continuously operated for a considerable length of time during the measurement, the aforementioned noise or drift causes a fluctuation in the baseline of the detection signal and thereby decreases the S/N ratio of the signal originating from the components of interest.